Our studies with PGHS-1 indicate that a Tyr radical is formed by the peroxidase activity of PGHS-1. Our data suggest that the Tyr radical is not the reactive intermediate that initiates arachidonic acid oxygenation. Recently we have shown that NO reacts with the tyrosyl radical converting the radical to a new radical characterized by ESR analysis. Reaction of the tyrosyl radical with NO does not alter the formation of prostaglandins and support the conclusion that the tyrosyl radical is not responsible for the removal of the 13-pro-R-hydrogen that initiates the cyclooxygenase reaction. These findings are being prepared for publication. Nitric oxide(NO) is reported to enhance prostaglandin formation by direct enhancement of PGHS activity. We found that NO is a substrate for the peroxidase but does not enhance the cyclooxygenase activity. Also, NO does not stimulate the expression of PGHS or alter LPS dependent PGHS-2 expression. We have also cloned rat PGHS-1 and -2 and studied their expression by rat tracheal cells (EGV-6). We observed an aberrantly sliced PGHS-1 mRNA in addition to the complete mRNA. Furthermore, we observed in these cells that TPA up-regulated PGHS-1 which in most cells is not upregulated by ligands but constitutively regulated. We have shown that EGF regulates the formation of 13-HODE in Syrian hamster embryo (SHE) cells.